High output CMOS-based image sensor

ABSTRACT

An image sensor has multiple light shields of conductive material, the light shields being formed with respective openings. A matrix array of unit cells are formed on a semiconductor body so that they respectively correspond to the light shields. Each unit cell includes a photosensitive region for receiving imagewise radiation through the opening of the corresponding light shield for producing photo-generated electrons, a floating diffusion region, and a transfer gate for transferring the photo-generated electrons from the photosensitive region to the floating diffusion region. An amplifying transistor is provided having a controlling terminal connected to the floating diffusion region for amplifying a potential developed in the floating diffusion region and a controlled terminal connected to the corresponding light shield. As a result, each light shield is biased at a potential that substantially varies with the potential of the floating diffusion region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to image sensors, and morespecifically to a CMOS-based image sensor.

2. Description of the Related Art

CMOS (complementary metal oxide semiconductor) based image sensors arecharacterized by low power consumption, operability on a single voltagesource (5 or 3.3 volts) and amenability to integration with peripheralcircuitry on a common chip, as described in a paper titled “A 128×128CMOS Active Pixel Image Sensor for Highly Integrated Imaging Systems”,Sunetra K. Mendis et al., IEDM 93, pages 583 to 586.

However, one disadvantage of the current CMOS-based image sensor is thatthe output voltage it generates is not of sufficient value.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aCMOS-based image sensor capable of producing a high voltage output.

The present invention is based on a recognition that there is a straycapacitance between a grounded conductive light shield covering portionsof the image sensor other than photosensitive areas and a conductorextending parallel with the light shield. It is found that the presenceof this stray capacitance is the source of the stated problem.

The problem is solved by segmenting the conventional light shield into aplurality of unit light shields respectively corresponding to unit cellsof the image sensor and biasing each unit light shield at a potentialwhich varies with the output voltage of the corresponding unit cell.

According to the present invention, there is provided an image sensorcomprising a plurality of light shields of conductive material, thelight shields being formed with respective openings. A matrix array ofunit cells are formed on a semiconductor body so that they respectivelycorrespond to the light shields. Each of the unit cells includes aphotosensitive region for receiving light through the opening of thecorresponding light shield for producing photo-generated electrons, afloating diffusion region, a transfer gate for transferring thephoto-generated electrons from the photosensitive region to the floatingdiffusion region, and an amplifying transistor having a controllingterminal connected to the floating diffusion region for amplifying apotential developed in the floating diffusion region and a controlledterminal connected to the corresponding light shield, so that the lightshield is ii biased at a potential that substantially varies with thepotential of the floating diffusion region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in further detail with referenceto the,accompanying drawings, in which:

FIG. 1 is a plan view of a prior art CMOS image sensor;

FIG. 2 is a cross-sectional view of a unit cell of the prior art CMOSimage sensor taken along the lines 2—2 of FIG. 1;

FIG. 3 is a plan view of a CMOS image sensor of the present invention;

FIG. 4 is a cross-sectional view of a unit cell of the image sensor ofthe present invention taken along the lines 4—4 of FIG. 3;

FIG. 5 is a cross-sectional view taken along the line 5—5 of FIG. 3; and

FIG. 6 is a cross-sectional view taken along the line 6—6 of FIG. 3;.

DETAILED DESCRIPTION

Before proceeding with the detailed description of the presentinvention, it may prove helpful to provide an explanation of the priorart with reference to FIGS. 1 and 2.

As shown in FIG. 1, the prior art CMOS image sensor consists of a matrixarray of pixel unit cells 100 and all the unit cells of the matrix arrayare overlaid with a metallic light shield 112. Each unit cell has aphotosensitive area, or photo-gate 102 and the light shield 112 has aplurality of openings for allowing external light to pass therethroughto the photo-gates 102 of the underlying unit cells. Each pixel unitcell of the prior art consists of a polysilicon photo-gate 102, atransfer gate 103 and a gate 104 of a reset transistor. Asource-follower amplifier transistor 105, a row (X) selection transistor106 and a source-follower load transistor 107 are connected betweenvoltage sources V_(DD) and V_(SS). The load transistor 107 is located atone end of each column of pixels, where two sample-and-hold circuits areprovided. One sample-and-hold circuit consists of a sampling transistor108 and a holding capacitor 110, the other sample-and-hold circuitconsisting of a sampling transistor 109 and a holding capacitor 111.Sampling transistor 108 is further connected to a column (Y₁) selectiontransistor, not shown, to produce a positive output signal and thesampling transistor 109 is further connected to column (Y₂) selectiontransistor, not shown, to produce a negative output signal. Note thatthe light shield 112 is connected to ground to be maintained at thereference potential.

The unit cell is fabricated on a common p-type semiconductor substrate201 on which a p-type well 202 and a lower insulating film 203 areformed. There is an upper insulating film 205 on the insulating film203, with the upper insulating film 205 slightly covering part of thephoto-gate 102 to allow the transfer gate 103 to partially overlap thephoto-gate 102. An n⁺-type floating diffusion region 206 and an n⁺-typeregion 207 are formed in the p-type well 202. The floating diffusionregion 206 is connected by a conductor 113 to the gate of row selectiontransistor 106 and the n⁺ region 207 is connected to the voltage sourceV_(DD) to function as the drain of the reset transistor. P+-type channelstoppers 208, which are grounded, bound the unit cell.

In operation, the photo-gate 102 is first supplied with a high voltagepulse to increase the potential of a depletion region that is formedunder this gate. During the signal integration period, photo-generatedelectrons are collected in the depletion region. The gate 104 of resettransistor is biased at a high voltage to act as a lateral anti-bloomingdrain and the row selection transistor 106 is biased off. Followingsignal integration, a row of pixels to be read out is addressed byenabling row selection transistor 106. Setting the photo-gate 102 aswell as the transfer gate 103 and the gate 104 of reset transistor at alow voltage then decreases the potential of the depletion region andlowers the potential barrier between the depletion region and thefloating diffusion region 206. This allows the photo-generated electronsto be emptied into the floating diffusion region 206, causing itspotential to rise. This potential variation is sensed and amplified bytransistor 105 and delivered to the output when the row selectiontransistor 106 is enabled. Although kTC noise is generated in thefloating diffusion region 206, the effect of this noise is eliminated bysampling the negative and positive signals using the sample and holdcircuits and taking the difference between the sampled values.

While the prior art solid state image sensor is compatible with the CMOSmanufacturing process, it has a disadvantage in that the voltageavailable from its output circuit is low.

According to the present invention, it is found that there is a straycapacitance C′ between the light shield 112 and the conductor 113. Thisstray capacitance C′ is an extra amount of capacitance which adds up tothe intentional capacitance C of the floating diffusion region 206.Since the voltage variation V of the floating diffusion region 206 isgiven by V=Q/(C+C′), where Q is the quantity of electrons stored in thediffusion region 206, the output voltage of the prior art image sensoris undesirably reduced as a result of the stray capacitance.

The image sensor of the present invention is shown in FIGS. 3 and 4,wherein parts corresponding in significance to those of FIGS. 1 and 2are marked by the same numerals as those in FIGS. 1 and 2. It is seen inFIG. 3 that the metallic light shield 112 of the prior art is dividedinto a plurality of light shields 200 which respectively overlies thecorresponding unit cells. Each light shield 200 has an opening 210 toexpose the photo-gate 102 to external imagewise radiation. In FIG. 4, itis seen that each light shield 200 is connected to the source oftransistor 105, instead of being to the ground terminal. As a result ofthis connection, a voltage variation is generated in each light shield200 by an amount equal to the source voltage multiplied by the gain ofthe source-follower amplifier transistor 105. This voltage variation isapproximately equal to the voltage variation of the floating diffusionregion 206.

If the conductor 113 is biased at a potential V and the source-followertransistor 105 has a gain a, the light shield 200 develops a voltage αV.Therefore, the voltage difference ΔV between light shield 200 andconductor 113 is equal to ΔV=V−αV=V(1−α). Since the value α is normally0.9, the voltage difference αV can be practically ignored. Accordingly,the undesirable effect of the stray capacitance between light shield 200and conductor 113 can be reduced by a factor (1−α), i.e., 0.1 times ascompared to the prior art unit cell. This results in an image sensorproducing an increased output voltage.

Since the light shields are separated from each other, there is apossibility that undesired light may intrude through the spacing betweenadjacent light shields and impinge on the photo-sensitive areas. Inorder to avoid this problem, horizontally elongated metallic shields 300are provided below the vertical spacing between adjacent light shields200 as shown in FIG. 5, and vertically elongated metallic shields 301are provided below the horizontally spacing between adjacent lightshields 200, as shown in FIG. 6. Light shields 200 and the additionallight shields 300 and 301 are electrically isolated from each other.

In the present invention, the light shields 300 and 301 areadvantageously used as metallic wiring. Particularly, the light shields300 are used as conductors for supplying reset pulse φR to the gates 104of the reset transistors of the same row. Light shields 301 are used forsupplying the voltage V_(DD) to all the unit cells.

What is claimed is:
 1. An image sensor comprising: a plurality of lightshields of electrically conductive material, said light shields beingformed with respective openings; a semiconductor body; a matrix array ofunit cells formed on said semiconductor body respectively correspondingto said light shields, each of said unit cells comprising: aphotosensitive region for receiving light through the opening of thecorresponding light shield for producing photo-generated electrons; afloating diffusion region; a transfer gate for transferring saidphoto-generated electrons from said photosensitive region to saidfloating diffusion region; and an amplifying transistor having acontrolling terminal connected to the floating diffusion region foramplifying a potential developed in the floating diffusion region and acontrolled terminal connected to the corresponding light shield, so thatthe corresponding light shield is biased at a potential whichsubstantially varies with the potential of said floating diffusionregion.
 2. The image sensor of claim 1, wherein said amplifyingtransistor is a source-follower transistor and said corresponding lightshield is connected to the source of the source-follower transistor. 3.The image sensor of claim 1, wherein the controlling terminal of saidamplifying transistor is connected to said floating diffusion region bya conductor extending parallel with the corresponding light shield. 4.The image sensor of claim 1, further comprising a plurality of elongatedmembers of electrically conductive material, each of said elongatedmembers being provided in such a location as to prevent external lightfrom intruding through a separation between adjacent ones of said lightshields.
 5. The image sensor of claim 1, further comprising a pluralityof elongated first members of electrically conductive material and aplurality of elongated second members of electrically conductivematerial, each of said first members being provided in such a locationas to prevent external light from intruding through a first separationbetween adjacent rows of said light shields and each of said secondmembers being provided in such a location as to prevent external lightfrom intruding through a second separation between adjacent columns ofsaid light shields.
 6. The image sensor of claim 5, wherein said firstmembers are arranged to supply a pulse to the unit cells of each row ofsaid matrix array.
 7. The image sensor of claim 5, wherein said secondmembers are arranged to supply an operating voltage to the unit cells ofeach column of said matrix array.
 8. The image sensor of claim 6,wherein said second members are arranged to supply an operating voltageto the unit cells of each column of said matrix array.